This invention relates generally to gas turbine engines and, more particularly, to fuel and control sub-systems for operating gas turbine engines with low heating value gaseous fuels.
Operating a gas turbine engine with a gaseous fuel which has a low heating value can adversely impact the fuel system due to poor ignition characteristics of low heating value fuels, a large volume of gas needed to sustain combustion, and a reduced flameout margin. To overcome the poor ignition characteristics, typically an alternate fuel is used for gas turbine engine start-up conditions.
To maintain acceptable fuel supply pressures with the large volume of gas needed to sustain combustion, the gas turbine engine includes a fuel nozzle discharge area that is significantly larger than a fuel nozzle discharge area for other fuel types. However, the increased size of the fuel nozzle discharge area provides a very low fuel nozzle discharge area pressure ratio during gas turbine engine start-up conditions using the alternate fuel which may lead to combustion instability and/or engine flameouts. Furthermore, rapid transient operations of the gas turbine engine, such as load drops, may result in additional engine flameouts due to a reduced flameout margin intrinsic with the low heating value fuel.
In an exemplary embodiment, a fuel and control sub-system is provided for operating a gas turbine engine with fuels having a low heating value. The fuel and control sub-system includes two separate metered gas fuel circuits and a dual gas nozzle. A main gas circuit is configured to deliver a fuel having a low heating value and a start gas circuit is configured to deliver at a start fuel having a high heating value to sustain the gas turbine engine during start-up and idle engine operation. The sub-system further includes a plurality of metering valves to transition the operation of the engine from operating with the start gas having a high heating value to operating with a main gas having a low heating value.
During operation, the control sub-system delivers the start gas, the main gas, or a mixture of start gas and main gas to the gas turbine engine. The sub-system also injects water into a combustor of the engine to decrease engine emissions. As a result, the control sub-system permits the user to adjust the fuel mixture to improve flammability limits and avoid regions of engine combustor instability. As a result, the control sub-system provides a user with flexibility to accurately control the delivery of fuel to a gas turbine engine.